Decoking plants have a drum which gradually fills with coke during the operation of a refinery. This coke must be removed when the drum is full. For this purpose a decoking tool is introduced into the drum from the top end of the drum. The decoking tool is suspended from a hollow drill stem that is coupled to a drive for causing the drill stem to rotate with the tool in the operating state. The drill stem is arranged on the above-mentioned crosshead and has a pressurized-water connection coupled to a pressure hose liftable and lowerable together with the crosshead for supplying the pressurized water to the drill stem. The length of the drill stem corresponds at least to the height of the drum to be emptied. The decoking tool can be traversed to the drum bottom by means of the drill stem.
The drum is emptied in such a manner that water exiting through nozzles arranged on the tool at high pressure of more than 100 bars supplied via the drill stem will selectively remove the coke which has been deposited in the drum.
During this emptying process the tool traverses the entire height of the interior of the drum along its longitudinal axis. To do this, the crosshead is lowered in a corresponding fashion by means of the cable of the winch and pulled upwards in the frame after emptying the drum until the decoking tool has been lifted out of the drum and is accessible on a work platform above the drum.
This emptying of the drum by means of high-pressure water jets from the decoking tool guided downwards on the drill stem in a rotary manner causes the crosshead to carry a heavy load due to the pressurized-water hose filled with pressurized water, the pressurized-water connection, the drill stem and its drive and the tool, in the order of approx. 3 to 5 tons depending on the size of the decoking plant. Moreover, the crosshead guided on guide rails of the profiles of the frame above the drum is exposed to knocks and vibrations while the drum is emptied. This is why the maintenance of the operability of the guiding components, which hold the crosshead mainly during lifting and lowering on the guide rails of the profiles of the frame, and in particular the brakes, is of particular importance. While regular checking and maintenance of the carrying and guiding components of the crosshead on site is possible to a certain extent, testing the continuing operability of the brakes has hitherto only been possible by removing them and testing and maintaining them in the workshop. It is difficult, however, to recognize when checking and maintaining needs to be carried out on the brakes of the crosshead. That is why this point in time is deferred for as long as possible as long as the brake components seem impeccable from the outside, without the certain knowledge whether the brakes will be able to intercept the crosshead according to their predetermined function and to fix it on the guide rails or, depending on the construction, on the profiles, and to thus prevent a complete fall of the crosshead in an emergency, namely if the carrying device of the crosshead fails and the crosshead falls, for example, due to a cable breaking.
If the crosshead should indeed fall, considerable damage arises due to the impact of the crosshead with all the attached components, such as the crosshead-side parts of the cable lifting device, the pressure hose mostly filled with pressurized water, and the drill stem together with the tool, the drive and the pressurized-water connection, which also damages the working platform and the top area of the drum itself, and moreover the life and health of the personnel working on the working platform are at considerable risk.
This lack of safety has been deemed acceptable. The accidents which have happened mostly due to the cables of the cable lifting device breaking and have led to the corresponding crosshead falling within the frame above the drum without the crosshead being intercepted by the brakes and fixed on the guide rails or on the profiles have not led to the brakes of the crosshead being removed and checked at regular intervals, mainly to avoid having to interrupt the operation which can lead to considerable disruptions in the operation of the refinery.